Answer
Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
It is not allowed for a woman to pray in a very thin garment. The prayer in such clothing is not correct.
Answering your question, the late Saudi scholar, Sheikh Ibn Baz (may Allah have mercy on him) stated:
It is not allowed for a woman to pray in a very thin garment. The prayer in such clothing is not correct. Instead, it is obligatory upon her to pray in clothing which is covering and through which one cannot see what is behind it or the color of her skin.
It is obligatory upon her to cover all of her body in the prayer, except for her face and hands. If she covers her hands also, that is better.
As for her feet, she must either cover them with socks that cover and conceal them or with clothing [a dress, for example] that rests over them.
Almighty Allah knows best.
Source: Islamic Fatawa Regarding Women, Compiled by Muhammad Ibn Abdul-Aziz Al-Musnad and translated by Jamal Al-Din Zarabozo, Darussalam, 1996.